170 DYNAimCAL GEOLOGY. 



feldspar to a hydrous alumina silicate is the cause. This finest of sediment 

 falls on incipient freezing (Brewer, 1883). Very thin particles, like scales 

 of mica, sink slowly, because the rate is that of particles (of the same 

 density) having a diameter equal to the thickness of the scales. They are 

 hence widely scattered by transporting waters. 



Transportation assorts in proi^ortion to size and specific gravity. — In 

 accordance with the ratio of transportation to velocity, it is found, supposing 

 the material to be alike in specific gravity, that a current of 4 miles an 

 hour will carry along stones 2^ inches in diameter ; of 2 miles, pebbles of 0-6 

 inch in diameter ; of |- mile, fine sand about 0-064 inch in diameter ; of -i- mile, 

 fine earth or clay, the particles 0-016 inch in diameter. Consequently, 

 materials will be arranged over the bottom by velocity of flow, the coarser 

 dropping first, the finer at greater or less distances beyond, and the finest 

 floating on to other places of deposition. 



Again, sands of like size but varying specific gravity will be assorted on 

 the same principle, iron sands (G = 5) being left behind where the ciirrent 

 is only sufficient to carry on garnet sand and other lighter kinds ; and garnet 

 sand (G = 3-6), where the quartz sand (G = 2-6) is still kept in move- 

 ment, so that several sorts of deposits may form by varying rates of flow. 

 If gold dust (G = 18 to 20) were in the waters, it would drop long before 

 the iron sand. The principle is used in ordinary gold washings. 



In drawing inferences as to rate of flow during deposition from the 

 fineness or coarseness of deposits, there is need of caution, because flowing 

 waters do not " scour " at the rates mentioned, unless the materials are quite 

 loose. Very slight compacting at surfaces will hold the sands and earth 

 down. Let any causes stir up the bottom, then the principle works well ; 

 and in these modern times steamers up and down rivers, bays, and coasts, 

 often occasion that stirring which favors scour, to the benefit of navigation. 

 Professor Verrill has remarked that the shells broken up by fishes over the 

 ocean's bottom make loose material easy of transportation by the Gulf 

 Stream. 



An important exception to this relation between size of particle^s and 

 hydraulic value, noticed and made the subject of special investigations by 

 E. W. Hilgard, arises from the tendency of the finer kinds of sediment in 

 fresh water, if the water is not absolutely quiet, to agglomerate their parti- 

 cles, when not over 1 mm. in diameter, into larger particles, or to flocculate, 

 as he terms the process, and so take the hydraulic value of coarser sediments. 

 He shows that fine river deposits consist largely of such, flocculated particles, 

 and that the fitness of soils for tillage depends largely on the porous condi- 

 tion thus derived. 



Some characteristics of water. — (a) A cubic foot of pure water at 62° F. weighs 

 436,495 grains, which equals 62-355 pounds, or nearly 1000 ounces avoirdupois = 28,315 

 grams. The soluble impurities of ordinary river water are 0-000186 of their weight. 

 (Murray.) 



Under a pressure of 1 atmosphere, water boils at 212° P. = 100° C. ; and under 45 

 atmospheres, at 510-6° F. = 265-9° C. 



